A pair of conductive lines are coupled when they are spaced apart, but spaced closely enough together for energy flowing in one to be induced in the other. The amount of energy flowing between the lines is related to the dielectric medium the conductors are in and the spacing between the lines. Even though electromagnetic fields surrounding the lines are theoretically infinite, lines are often referred to as being closely or tightly coupled, loosely coupled, or uncoupled, based on the relative amount of coupling. The amount of coupling may be defined by a coupling coefficient. However, as a practical measure, two lines may be considered to be inductively coupled when a detectable signal is coupled from one line onto the other. A threshold of coupling may be appropriate to distinguish between coupled and uncoupled lines. In most applications, two lines that have less than 20 dB inductive coupling between them are considered to be uncoupled lines. In some applications, lines that have less than 100 dB are considered to be uncoupled lines. In terms of a coupling coefficient, two lines may be considered to be closely coupled if the coupling coefficient is 0.1 or greater. Thus, two lines may be considered as loosely coupled or substantially uncoupled if they have a coupling coefficient of less than 0.1.
Couplers are electromagnetic devices formed to take advantage of coupled lines, and may have four ports, one associated with each end of two coupled lines. A main line has an input connected directly or indirectly to an input port. The other end is connected to the direct port. The other or auxiliary line extends between a coupled port and an isolated port. A coupler may be reversed, in which case the isolated port becomes the input port and the input port becomes the isolated port. Similarly, the coupled port and direct port have reversed designations.
A hybrid coupler is generally assumed to divide its output power equally between the two outputs. One type of hybrid coupler is referred to as a ring-hybrid coupler, such as the hybrid coupler disclosed in U.S. Pat. No. 3,516,025. This device is a four port hybrid formed of two pairs of ports such that the opposite ports of a pair are isolated from one another and each port is closely coupled to the ports of the other pair. This hybrid coupler includes three equal length sections of transmission line with terminating loads connected across both ends of each of the transmission lines. One conductor of each of the transmission lines is also connected at both ends to ground. A fourth equal length section of transmission line connects the free ends of two of the transmission lines with the connections at one end of this fourth transmission line being reversed. The lengths of each of the transmission lines are selected to be one quarter of a wavelength for the center frequency of the bandwidth over which the hybrid is to operate.
Such a conventional ring-hybrid may include in series with each terminating load a transmission line of length equal to the length of the transmission lines in the ring and of a selected characteristic impedance. This quarter wavelength line is left open at the unconnected end. The limitations of bandwidth experienced in the ring-hybrid arise at frequencies below the center frequency because of an inherently inductive characteristic, whereas the limitations in bandwidth at frequencies above the center frequency arise, because at these frequencies, the network appears inherently capacitive. The open quarter wavelength sections tend to compensate for this effect since at increased frequencies they appear inductive and at decreased frequencies they appear capacitive. The network can be further compensated by including in series with each terminating load a quarter wavelength open ended section of transmission line and also in shunt with each load a quarter wavelength shorted end section of transmission line.